Hepatic Histology

Basic Components of Hepatic Histology

Hepatocytes:

Hepatocytes carry out the synthetic and metabolic functions of the liver. They occupy the majority of the liver and exist in anastomosing plates of cells. These plates are just two cells thick in between which run small bile canaliculi (see below).

Sinusoidal Capillaries (AKA "Hepatic Sinusoids")

Sinusoidal Capillaries are an anastomosing network of capillaries, lined by a specialized, discontinuous fenestrated endothelium which run in between the plates of Hepatocytes. The Sinusoidal Capillaries do not have a basement membrane, allowing blood to directly contact the Hepatocytes.

Space of Disse

The space which lies in between the endothelium of the sinusoidal capillaries and the Hepatocytes is termed the "Space of Disse". A special type of macrophage termed the Kupffer cell lives in this space along with Ito Cells (AKA "Stellate Cells") which store Vitamin A.

Terminal Hepatic Venule

Terminal hepatic venules are relatively large venules that are interspersed in a regular, nearly hexagonal pattern through out the liver. Blood from the sinusoidal capillaries ultimately drains into these venules.

Portal Tracts:

Portal Tracts are thin areas of fibrous tissue through which run the portal triads. Portal tracts are interspersed throughout the liver in a regular nearly hexagonal pattern around each terminal hepatic venule.

Portal Triads:

Portal Triads refer to the smallest branches of the hepatic artery, portal vein, and biliary tree (i.e. Bile Ductule), which travel together in a triad configuration within the portal tracts. Blood from the branches of the hepatic artery and portal vein travel into the sinusoidal capillaries toward the terminal hepatic venule while bile generated by hepatocytes travels in the opposite direction through bile canaliculi into the bile ductules of the triad.

Hepatic Lobule

Overview

The histological architecture of the liver can be thought of as repetitive units termed "Liver Lobules". The liver lobule can be conceptualized in different ways and each conceptualization highlights a different aspect of liver architecture, function, and disease pathogenesis.

Classic Hepatic Lobule

The Classic Hepatic Lobule is a roughly hexagonal structure at the center of which lies the terminal hepatic venule (AKA: Centrilobular Venule). The six angles of the hexagon are occupied by portal tracts through which run the portal triads. The rest of the lobule is occupied by anastomosing plates of hepatocytes in between which lie specialized sinusoidal capillaries. Again, blood exiting the arterial and portal vessels of the portal tracts run through the sinusoidal capillaries to the terminal hepatic venule while bile runs in the opposite direction, through the bile canaliculi, to the bile ductule in the portal triad. The "Classic Hepatic Lobule" conceptualization highlights the most visibly obvious architectural organization of the liver.

Portal Lobule

The portal lobule can be thought of as an equilateral triangle whose angles correspond to three adjacent terminal hepatic venules at the center of which lies a portal triad with its bile ductule. The portal lobule highlights the bile-synthetic functional unit of liver as all of the bile synthesized by the hepatocytes within the "Portal Lobule" drains to a common bile ductule.

Liver Acinus

The Liver Acinus is defined as a ovoid area whose long axis corresponds to two adjacent terminal hepatic venules and whose short axis corresponds to two adjacent portal triads. The liver acinus conceptualization highlights the course of blood flow within the liver. Those areas closest to the portal triads receive blood the earliest and those near the terminal hepatic venule receiving blood the last. How blood flow travels through the liver is important because the concentration of oxygen and toxins within the liver changes as it travels from the portal tracts to the terminal hepatic venule. These changes occur due to the metabolic activity of hepatocytes which consumes oxygen and degrades toxins. Based on the liver acinus conceptualization, pathologists have designated three zones corresponding to different degrees of oxygenation:

Zone 1 surrounds the portal triads, has the earliest contact with oxygen-rich and potentially toxin-rich blood.

Zone 2 lies between the portal triads and terminal hepatic venules, and has contact with oxygen-intermediate and potentially toxin-intermediate blood.

Zone 3 surrounds the terminal hepatic venules, is the last to contact blood which is by this point oxygen-poor and lowest in toxin concentration, if present.